In the papermaking process, a papermaking machine is conventionally equipped with a wire part, a press part and a dryer part for squeezing water from a wet paper web. The wire part, press part and dryer part are arranged in this order in the transfer direction of the wet paper web. The wet paper web is squeezed while being transferred by passing from one papermaking equipment to the next provided in the wire part, the press part and the dryer part, and is finally dried in the dryer part.
In these parts, papermaking equipment is used which corresponds to the functions of dewatering the wet paper web (wire part), squeezing water from the wet paper web (press part) and drying the wet paper web (dryer part). Moreover, the press part is generally equipped with one or more press devices arranged in series next to each other in the direction in which the wet paper web is transferred.
In each press device, an endless felt (closed type) or an open-ended felt that has been formed into an endless felt by connecting it in the papermaking machine is provided. Each press device also comprises a pair of rolls, which face each other (namely, a roll press), or a roll and a shoe press; the wet paper web is placed on the felt, and, while it is moving together with the felt in the wet paper web transfer direction, moisture is squeezed from the wet paper web by pressing the wet paper web together with the felt and the shoe press belt in the roll press or in the shoe press; the moisture pressed from the wet paper web is continuously absorbed by the felt or passes through the felt to be discharged to the outside of the felt.
Hereinafter, one example of the above-mentioned press device part will be described with reference to FIG. 5. By using a shoe press mechanism in which a shoe press belt 2 in loop shape is interposed between a press roll 1 and a shoe 5, dewatering is performed by passing a transfer felt 3 and a wet paper web 4 in the press portion formed by the press roll 1 and the shoe 5.
As shown in FIG. 2, the shoe press belt 2 is configured by providing an outer circumferential polyurethane layer 21 and an inner circumferential polyurethane layer 22 on both sides of a fibrous base material 6 which is sealed (embedded) in the polyurethane layers; wherein moreover a plurality of concave grooves 24 is formed in the surface of the press roll-side outer circumferential polyurethane layer 21, and the water wrung from the wet paper web 4 during the pressing described above is retained in the concave grooves 24, so that the detained water is further removed to the outside of the pressing portion by the rotation of the belt. For this reason, convex parts 25, provided on the press roll-side outer circumferential polyurethane layer 21, are required to have improved wear resistance, crack resistance, flexural fatigue resistance and other mechanical characteristics vis-à-vis the pressing force in the perpendicular direction applied by the press roll 1 as well as in relation to the wear and flexural fatigue of the shoe press belt occurring in the shoe press region.
For these reasons, polyurethane having good crack resistance is widely used as resin material for forming the outer circumferential polyurethane layer 21 of the shoe press belt 2.
JP, A, 2002-146694 (Patent Document 1), for example, proposes a shoe press belt made from an integrated structure of a reinforcing fibrous base material and polyurethane, the polyurethane comprising an outer circumferential layer and an inner circumferential layer, the reinforcing fibrous base material being embedded in the polyurethane, wherein
a polyurethane of the outer circumferential layer is a polyurethane with a “JIS A hardness” of 89 to 94 made by curing mixed composition of                a urethane prepolymer (manufactured under the trade name of Hiprene L by Mitsui Chemicals, Inc.) having a terminal isocyanate group and obtained by reacting tolylene-2,6-diisocyanate (TDI) with polytetramethylene glycol (PTMG), and        a curing agent (also called chain extension agent) containing dimethylthiotoluene diamine,        in which the urethane prepolymer and the curing agent are mixed such that the equivalent ratio (H/NCO) of the active hydrogen group (—H) of the curing agent and the isocyanate group (—NCO) of the urethane prepolymer is in the range of 1<H/NCO<1.15, and        
a polyurethane of the inner circumferential layer is a polyurethane made by curing a mixed composition of                a urethane prepolymer having a terminal isocyanate group and obtained by reacting 4,4′-methylenebis(phenylisocyanate) (MDI) with polytetramethylene glycol (PTMG), and        a curing agent mixture of 65 parts of dimethylthiotoluene diamine and 35 parts of polytetramethylene glycol (PTMG),        in which the urethane prepolymer and the curing agent are mixed such that the equivalent ratio (H/NCO) of the active hydrogen group (H) of the curing agent and the isocyanate group (NCO) of the urethane prepolymer is in the range of 0.85≦H/NCO<1.        
JP, A, 2002-146694 (Patent Document 1) further proposes a papermaking process belt made from an integrated structure of a reinforcing base material and thermosetting polyurethane, the reinforcing base material being embedded in the polyurethane, the outer circumferential surface and the inner circumferential surface being made from the polyurethane, wherein a polyurethane, which forms the outer circumferential surface, is made from polyurethane of a composition comprising a urethane prepolymer having a terminal isocyanate group and a curing agent containing dimethylthiotoluene diamine.
JP, A, 2008-285784 (Patent Document 2), moreover, proposes a shoe press belt, shown in FIG. 1, comprising a reinforcing fibrous base material 6, embedded in polyurethane, an outer circumferential layer 2a and an inner circumferential layer 2b, each made of polyurethane, wherein the outer circumferential layer is made from a polyurethane comprising a polyurethane layer comprising
an urethane prepolymer (A) produced by reacting an isocyanate compound selected from p-phenylene-diisocyanate and 4,4′-methylenebis(phenylisocyanate) with a polytetramethylene glycol (PTMG) and having a terminal isocyanate group, and
a curing agent mixture (B) comprising 1,4-butanediol and an aromatic polyamine having an active hydrogen group (H).
Compared to the shoe press belt according to Patent Document 1, the shoe press belt according to Patent Document 2 uses a straight chain polyol compound as polyol component and an isocyanate compound selected from p-phenylene-diisocyanate and 4,4′-methylenebis(phenylisocyanate) of which the hardness, flexural resistance and curing speed as polyisocyanate of the polyurethane material are difficult to adjust; therefore, it has the excellent properties such as resistance against flexural fatigue, resistance to crack propagation, resistance to groove closure, hardness, elongation properties and toughness of the wear characteristics.
JP, T, 2007-530800 (Patent Document 3), moreover, proposes a papermaking process belt having a polyurethane layer comprising a coating which has polyurethane, as its base, using TDI and MDI as isocyanate compounds and including an amount from about 0.01 to about 10% by weight, preferably 1 to 5% by weight, of nanoparticles ranging in lengths from about 100 nm to about 500 nm but not exceeding an average size distribution of 100 nm. Patent Document 3 mentions that this papermaking process belt improves at least one of the following characteristics: resistance to flex fatigue, resistance to crack propagation, resistance to groove closure, hardness, elongation characteristics and wear characteristics.
The specification of Japanese Patent No. 3264461 (Patent Document 4) is related to a transfer belt (conveyor belt) and discloses a transfer belt used in a papermaking or paperboard-making machine and the like for carrying a web from a first transfer point, where the transfer belt is subjected to compression, in a closed draw to a second transfer point. The transfer belt comprises a reinforcing base fabric and an aliphatic polyurethane polymer coating on the paper side (outer layer) of the reinforcing base fabric, wherein
the reinforcing base fabric has a back side and the above-mentioned paper side,
the polymer film is formed by coating and drying an aliphatic polyurethane aqueous dispersion liquid comprising 23.6% by weight of kaolin clay and 67.5% by weight of aliphatic polyurethane (solid volume) on the reinforcing base fabric surface, and has a hardness ranging from Shore A 50 to Shore A 90,
the polymer film comprises a web-contacting surface with a pressure-responsive recoverable degree of roughness,
the roughness before the polymer film is compressed is in the range from Rz=2 microns to 80 microns,
when the transfer belt is in the press nip, this roughness is in the range from Rz=0 micron to 20 microns, and
after exiting the press nip, it is capable of returning to the roughness it had before the compression.
Patent Document 3 lists clay, carbon black, silica, silicon carbide, or metallic oxides such as alumina as examples of nanoparticles. As examples of metallic oxides are listed aluminum oxide, titanium oxide, iron oxide, zinc oxide, indium oxide, tin oxide, antimony oxide, cerium oxide, yttrium oxide, zirconium oxide, copper oxide, nickel oxide and/or tantalum oxide and combinations thereof. For example, in one embodiment, up to 1% by weight of uncoated alumina, alumina coated with epoxysilane or octylsilane was added. It is further mentioned that clays may include montmorillonite such as Cloisite (registered trade name) 30B, saponite, hectorite, mica, vermiculite, bentonite, nontronite, volkonskoite, manadiite and henyaite and combinations thereof.
The papermaking process belt according to Patent Document 3 has a higher surface hardness than the transfer belt disclosed in Patent Document 4; moreover the figures for resistance to flex fatigue and resistance to crack propagation disclosed in its specification are about 4 to 5 times better than those for the papermaking process belt according to Patent Document 1; however, resistance to flex fatigue and resistance to crack propagation of the papermaking process belt according to Patent Document 3 are inferior to the corresponding figures for the shoe press belt according to Patent Document 2.
The shoe press belt according to Patent Document 2 uses an isocyanate compound selected from p-phenylene-diisocyanate and 4,4′-methylenebis(phenylisocyanate); therefore, there is the disadvantage that it is difficult to control the temperature when the polyisocyanate compound and the curing agent are heated.
The process belt of Patent Document 4 uses an aliphatic polyurethane aqueous dispersion liquid comprising 23.6% by weight of kaolin clay and 67.5% by weight of aliphatic polyurethane (solid volume) as coating agent; therefore, there is an increase in the “JIS A hardness” of the belt due to the kaolin clay and an improvement of the extensibility of the belt due to the aliphatic polyurethane; however, the improvement of the durability is still insufficient.